The removal of contaminants, such as paint and pigeon droppings on substrates such as metals, plastic, painted surfaces, wood, concrete and stone, is accompanied by damage to the substrate surfaces when mechanical methods such as blasting with solid particles, are used.
In many cases the use of acid, bases or cleaning agents containing organic solvents for cleaning is not possible because they attack the substrate surface.
Cleaning with cold or hot water or steam, optionally under pressure, leads, in particular in the case of open-pore substrates, such as building materials, to severe loading of the substrate with water, with consequent damage.
Diverse known methods facilitate easier cleaning by means of a preventive treatment of the substrate surfaces.
It is known in practice to treat the substrate surfaces with a concentrated solution of organic polymers. These treatments, which are termed sealing, are ineffective and in the case of porous substrates block the pores, so that the permeability to air and water vapor is prevented. The sealings frequently also lead to adverse optical changes in the substrate surfaces, such as shine or dark discoloration. The solutions of organic polymers which are used in practice comprise predominantly organic solvents, which are a hazard to health and a fire hazard and pollute the environment. The removal of contaminants from substrate surfaces pretreated in this manner can usually be effected only by means of organic solvents.
An oil- and water-repellent impregnation which hinders the permeability to air and water vapor to a lesser extent is described in DE-A-25 26 287 (M. Roth and H. Gluck; Wacker-Chemie GmbH, laid open on Dec. 30, 1976). With this method, the substrate surfaces are treated with a combination of organosilicon compounds and fluorine-containing organic compounds. However, the removal of contaminants requires the use of organic solvents.
A method for impregnating stone is disclosed in DE-C-36 30 520 (H. Ramesohl; published on Dec. 17, 1987), in which a detachable coating is applied which is receptive to paints but does not penetrate the impregnated stone. In the illustrative embodiment, the stone surface is impregnated to render it oil- and water-repellent and a coating composed of hard waxes liquefied by solvents is applied thereon. However, the wax coating impairs the permeability to air and water vapor to such an extent that the drying characteristics of the stone treated by this method are impaired, which can lead to damage. In addition, the detached wax, is a substance which pollutes the environment and must be disposed of as special waste.
It is also known in practice to apply aqueous solutions of polysaccharides to the substrate surface and to rinse off this coating, as required, together with the contaminants using water. This method has not proved suitable on outdoor surfaces exposed to rain, in particular, because of the solubility in water of the polysaccharide coating. The polysaccharide coating is neither water-repellent nor oil-repellent and does not protect the substrate against the uptake of water or organic liquids, such as fats and oils.
Therefore, it is an object of the present invention to provide a method for coating substrate surfaces which are not water-absorbent. Another object of the present invention is to coat substrate surfaces with a coating which is permeable to air and water vapor and is adequately resistant to weathering. Still another object of the present invention is to coat substrate surfaces with a coating which is easily removed, residue-free, together with the contamination without organic solvents. A further object of the present invention is to coat substrate surfaces with a coating which neither displays a shine nor dark discoloration. A still further object of the present invention is to provide a method for coating porous substrates to impart water-repellent properties thereto.